Urea is commercially obtained by reacting NH.sub.3 and CO.sub.2 in a reactor at elevated temperature and pressure. At urea synthesis conditions NH.sub.3 and CO.sub.2 instantaneously and completely react stoichiometrically to form ammonium carbamate. The latter is partially converted to urea and water within 20-30 minutes of residence in the reactor. Ammonia in excess of the stoichiometric ratio with CO.sub.2 is used in the reactor for the purpose of increasing the conversion of carbamate to urea. The urea synthesis reactor usually contains urea, water, excess ammonia and unconverted ammonium carbamate. For the purpose of separating the finished urea solution product from the unconverted reactants, the urea reactor effluent is usually let down in pressure through a reducing valve and heated in a decomposer, usually operated at a constant pressure, and thus the unconverted ammonium carbamate is decomposed to gaseous NH.sub.3 and CO.sub.2, and the gas is expelled from the urea product solution together with the excess ammonia evaporated from the latter with some water.
The NH.sub.3 and the CO.sub.2 gas with water vapor thus recovered is usually absorbed in water to form an aqueous ammoniacal solution of ammonium carbamate, and the solution thus obtained is recycled back to the urea synthesis reactor for total recovery.
The water contained in the recycle carbamate solution is detrimental to the conversion of ammonium carbamate to urea in the urea synthesis reactor. In order to minimize the adverse effect of the water upon the conversion of carbamate to urea in the reactor, the water content in the carbamate recycle solution is usually maintained at the lowest possible value. This minimum value is dependent upon the ammonium carbamate content of the carbamate recycle solution and consequently upon the salting-out temperature of carbamate. The water content in the carbamate recycle solution is usually maintained at such a level as to prevent the salting out of ammonium carbamate from the solution recycled to the reactor.
Thus, in practice, a continuous control of the water content in the carbamate recycle solution is required in order to prevent either the urea synthesis reactor from operating inefficiently, or to prevent crystallization of the carbamate recycle solution with obvious consequent mechanical problems in the equipment handling the carbamate recycle solution.
In practice such a continuous control of the water content in the carbamate recycle solution is difficult to achieve, because of the methods that are normally used to determine the water content in the carbamate recycle solution.
Analytical methods and various other methods are normally used to determine the concentration of the carbamate recycle solution.
One of the most commonly used methods is the analytical method, which mainly consists of taking a sample of the carbamate recycle solution and analyzing it in the laboratory for a complete determination of the ammonium carbamate expressed as equivalent CO.sub.2, of the equivalent ammonia as ammonium carbamate and the free ammonia dissolved in water both expressed as total ammonia. The water content is obtained by difference, by substracting the determined values from the carbamate recycle solution.
The drawbacks of such an analytical method are obvious. Due to the usual minimum time lag of 6-7 hours between the time of the sampling of the carbamate recycle solution and the time the results of the laboratory analyses are available to the urea plant operator for the purpose of deciding how much water should be added to the carbamate recycle solution, or due to the several inherent possible sources of error in sampling and analysis of the carbamate recycle solution, efficient control cannot be realized.
Various other methods and techniques have been suggested in the past for a more or less continuous determination of the carbamate recycle solution concentration. These other methods consist mainly of monitoring one of the following four specific physical properties of the carbamate recycle solution: density, viscosity, electrical conductivity, or the specific velocity of sound through it.
These four basic methods based on the measurement of one of the specific physical properties of the carbamate recycle solution listed above, besides being subject to mechanical problems due to the delicate moving parts of the measuring apparatus in contact with the very corrosive carbamate recycle solution, have had the common problem of being relatively inaccurate and inconsistent due to a common cause. This common cause is due to the fact that each of the three components of the carbamate recycle solution, namely ammonium carbamate, free ammonia and water, has quite a different specific gravity, viscosity, electrical and sound conductivity. The difference between these four physical properties is relatively large especially for water and free ammonia. For example the specific gravity of water is one, and the specific gravity of liquid ammonia, when mixed with water, is about 0.71. The viscosity of water is equal to one centipoise, and that of liquid NH.sub.3 is equal to about 0.16 centipoise. A similar relative difference exists between the other two physical properties of water and ammonia.
It is obvious that at the same ammonium carbamate to water ratio, the four physical properties of such a solution described above, namely specific gravity, viscosity, electrical and sound conductivity, are susceptible to the slightest variation in free NH.sub.3 content of the carbamate recycle solution.
Due to the fact that the crystallization point of the carbamate recycle solution depends primarily upon the ratio of the dissolved ammonium carbamate to the water contained in the solution, and to a very minor degree depends upon the ratio of the dissolved free ammonia to the water contained in the solution, it is obvious that a relative variation in the crystallization point of the carbamate recycle solution can not be reliably detected by measuring one of the above mentioned four physical properties of the carbamate recycle solution.
In the above case, a relative variation in one of the above mentioned four physical properties of the carbamate recycle solution can be interpreted either as a variation in the crystallization point of the carbamate recycle solution due to a variation in the ratio of the dissolved ammonium carbamate to the water contained in the solution, or as a variation in the ratio of the dissolved free ammonia to the water contained in the solution without any consequent appreciable variation in the crystallization point of the carbamate recycle solution.
In a specific case wherein the physical property under observation, namely either the specific gravity, viscosity, electrical or the sound conductivity, remained relatively constant but at the same time the ammonium carbamate to water ratio and consequently the crystallization point of the carbamate recycle solution for instance increased by the same equivalent value of the particular physical property under observation, by which the free ammonia to water ratio decreased, it has been found that the crystallization point of the carbamate recycle solution and thus its concentration changed more or less drastically, but this relative change failed to be detected by observing the particular and above mentioned physical property under consideration, because the physical property under observation for the total carbamate recycle solution did not vary. In this case the relative increase in the value of the physical property under observation is counteracted by the equivalent decrease of the physical property under observation for the exactly same relative value, so that in effect no change of the physical property of the whole carbamate recycle solution is registered.
Various other methods of controlling the concentration of the carbamate recycle solution were suggested in the past, as for instance in the U.S. Pat. No. 3,270,050. A drawback of the process disclosed in U.S. Pat. No. 3,270,050 is the fact that the by-product biuret content in a urea product solution varies according to the temperature variations which occur in a degasser-evaporator.
Another drawback of the above mentioned process is the fact that the relationship between the concentration of a concentrated aqueous ammonium carbamate solution and its viscosity is not consistent.